Apparatus and method for investigating a sample

ABSTRACT

An apparatus for investigating a sample comprising a source of a beam of radiation, a detector for detecting a beam of radiation reflected by or transmitted through a sample to be imaged, an optical subsystem for manipulating the beam between the source and detector and means for translating the optical subsystem along a first translation axis relative to the source and detector to scan the beam across the sample, wherein the source and the detector are on opposite sides of the subsystem and the beam from the source and the beam reflected or transmitted each enter and exit the subsystem in a direction parallel to the first direction of translation. The apparatus is also suitable for maintaining the relative phase of two beams of radiation, during translation of an optical subsystem.

[0001] This invention relates to the field of investigating and imagingsamples. More specifically, the invention is concerned with suchapparatus which scan a beam of electromagnetic radiation relative to thesample. The present invention is primarily intended for use in theso-called terahertz regime, this is typically thought of as being therange of frequencies from approximately 25 GHz to 100 THz.

[0002] It is known to use terahertz radiation to obtain spectra andimages of samples. In particular, EP-A-0 727 671 discloses the use of apulsed beam of terahertz radiation to investigate a sample. The pulsedbeam comprises a plurality of different frequency components.

[0003] Materials, in general, have a frequency dependent response toradiation. By analysing the frequency components of the terahertzradiation in the time domain or frequency domain, an image of the samplecan be generated.

[0004] While the present invention is suitable for use to obtain spectrafrom a number of spatially distinct points on a sample, it is primarilyconcerned with the production of images of a sample. An image of asample can be generated by scanning the sample relative to the focus ofa beam. In the case of a large sample, it is not practical to move thesample, thus the focus of the beam must be scanned instead.

[0005] This has been alluded to in EP-A-0 727 671 which disclosesfocusing a signal source on distant points and moving the sample ormoving the source and detector across the sample. However, no indicationis given as to how this may be achieved. Due to the manner of operationof such an imaging system, it is not apparent how the radiation may bescanned over a sample, while still being able to obtain meaningfulresults. PCT/GB00/00632, by the present applicant, discloses a methodand apparatus for imaging a sample using terahertz radiation. Thepresent invention builds on this and offers increased accuracy forimaging.

[0006] When considering the problems with scanning the beam relative tothe sample it is important to understand what happens when a sample isirradiated by radiation. Electromagnetic radiation is a wave phenomenaand hence it is characterised by both amplitude and phase. Thusinformation regarding the material of the sample, and its internalmake-up can be found by measuring the change in amplitude and phase ofthe radiation introduced by the sample at different frequencies. Thephase is related to the propagation path length or propagation time ofthe radiation.

[0007] When the sample is scanned through the beams, the beams and theirassociated optics do not move, thus it is possible to easily measure thephase change caused by the sample. However, once the beam itself isscanned, problems arise in how to accurately determine the phase shiftintroduced by the sample.

[0008] It is an aim of the present invention to alleviate, or partiallymitigate some of the problems associated with the prior art. Inparticular, it is an aim of the present invention to provide anapparatus which allows scanning of a beam of radiation over a sample inat least one dimension while taking account of any change in time delayintroduced by the system itself, so that the delay introduced by thesample may be found.

[0009] According to a first aspect of the invention, there is providedan apparatus for investigating a sample, comprising a source of a beamof radiation, a detector for detecting a beam of radiation reflected byor transmitted through the sample, an optical subsystem for manipulatingthe beam between the source and detector, and means for translating theoptical subsystem along a first translation axis relative to a fixedreference point to scan the beam across the sample, wherein the beamfrom the source enters the subsystem on one side of the subsystem in adirection parallel to the first translation axis, and the beam reflectedor transmitted exits the subsystem on the opposite side of the subsystemin a direction parallel to the first translation axis.

[0010] The subsystem comprises at least one element for manipulating thebeam or pulse of radiation which may be translated along a translationaxis relative to sample. Generally, the subsystem will comprise aplurality of elements which may be translated together in unison alongthe desired translation axis. Preferably, the subsystem will comprise atleast one element configured to direct radiation onto the sample and atleast one other element configured to direct radiation reflected fromthe sample into the detector.

[0011] The beam enters and exits the subsystem in the same direction andparallel to the axis of translation and the subsystem is moveable alongthe translation axis. Thus, the path length travelled between the sourceand detector is constant regardless of the scanning position of thebeam. Specifically, if the subsystem is moved away from the source, theextra distance travelled between source and subsystem is compensated bythe shortening of the distance travelled between subsystem and detector.

[0012] The above arrangement keeps the path length of the irradiatingradiation constant regardless of the scanning position of the beam.Thus, providing that the detector is provided with some informationconcerning the phase of the radiation leaving the source, the phasedifference introduced by the sample can be determined.

[0013] The detector may be given information about the phase of theradiation leaving the source by a number of different methods. Forexample, the source and detector may both be provided with asynchronised clock signal; Preferably, a reference beam is used whichdoes not pass through the sample and which has a phase related to thatof the beam of irradiating radiation.

[0014] In apparatus according to the above aspect of the presentinvention, the path length of the reference beam will be fixed as thepath length of the irradiating radiation is fixed and a known phasebetween the two beams can be maintained. However, it is also possible todesign a system where the path length of the irradiating radiationchanges, but where the path length of the reference beam changes by acorresponding amount This arrangement still allows the detector tomeasure the change in phase between the irradiating beam and referencebeam.

[0015] Thus, in a second aspect, the present invention provides anapparatus for investigating a sample, comprising a source of a beam ofradiation, a source of a reference beam, an optical subsystem formanipulating the source beam means for translating the optical subsystemalong a first translation axis relative to the sample to scan the beamacross the sample and a detector for detecting the reflected ortransmitted beam wherein source and reference beams each enter thesubsystem in a direction parallel to the first translation axis.

[0016] As the subsystem moves, the relative phase of the source andreference probe as they enter the subsystem will stay the same so thatthe phase relationship between the two beams is altered only by thesource beam impinging on the sample. In this aspect, the reference beamis not stationary relative to the detector, but rather to the source.This allows accounting for the phase change introduced by the apparatusas it moves to be made automatically, and allows a comparison of thesource and reference beams at the detector to detect the phase changecaused by the delay introduced by the sample. This can be done at manyfrequencies, the phase change at each frequency not necessarily beingthe same, due to the frequency dependent refractive index of the sample.

[0017] The sample is preferably placed independent of the subsystem sothat the sample may remain stationary while the subsystem moves themanipulating elements relative to the sample.

[0018] The above description has concentrated on systems which scan injust one direction However, it is possible to put two such systemsinside one another in order to scan in two or more directions.

[0019] Thus, preferably, a second optical subsystem is provided formanipulating the source beam between the source and the detector, thefirst subsystem being contained within the second subsystem. Preferably,there are means for translating the second optical subsystem relative tothe sample, to scan the source beam across the sample along a secondtranslation axis, wherein the source beam enters the second subsystem ina direction parallel to the direction of translation of the secondsubsystem.

[0020] Preferably, the first and second translation axes are notparallel, so that a focus of the beam may be scanned in two dimensionsand a two dimensional image of the sample may be constructed. However, asystem where there are two subsystems which scan in the same directionmight be useful. For example, the first subsystem may have a limitedmovement range but have fine control whereas the second subsystem has amuch longer movement range, but which cannot be so finely controlled.

[0021] The second optical subsystem may be in accordance with the secondaspect of the present invention, where the second optical subsystemcomprises the detector. The reference beam and source beam entering thesecond subsystem parallel to a second translation axis relative to thesample.

[0022] The detector may be outside the second subsystem in the manner ofthe first aspect of the present invention. If this is the case then thereference beam and reflected or transmitted beam exit the secondsubsystem parallel to the direction of translation of the secondsubsystem. The reference beam and the reflected or transmitted beam bothtravel the same distance to the detector regardless of the position ofthe subsystems. Therefore, the phase shift in the reflected ortransmitted beam, introduced by the sample, relative to the referencebeam is maintained until the beams reach the detector.

[0023] The first subsystem which is inside the second subsystem willgenerally be of the type described with reference to the first aspect ofthe present invention where the detector lies outside the subsystem.

[0024] Preferably the first and second directions of translation areorthogonal. This gives the advantage that the first and secondsubsystems can move independently, movement along one translation axisnot causing any movement along the other translation axis.

[0025] However, the first and second translation axes need not beorthogonal for the invention to operate. The beam irradiating the samplemay be incident on the sample at any angle, although in general the beamis incident on the sample along a direction largely orthogonal to thefirst and second translation axes.

[0026] It is possible to incorporate first and second subsystems asdescribed above into a third subsystem which moves along a thirdtranslational axis. This can be done by arranging the source outside thethird subsystem and the source beam, the third subsystem parallel to thethird translational axis. The third subsystem would manipulate the beamsso as to enter the second subsystem parallel to the second translationalaxis. In this case, the source and reference beams would each travel thesame distance, the change in distance due to movement of the second andthird subsystems will be the same for each beam. The apparatus will notintroduce any phase difference between the two beams when each subsystemis moved along its translational axis. Alternatively, the source andreference source could be mounted within the third subsystem and movedtogether along the third translation axis.

[0027] The above description has generally referred to systems where thebeams of radiation (e.g. the irradiating beam of radiation or thereference beam) pass through free space and are reflected and focused byfree space optics. However, the benefits of the first and second aspectsof the present invention can also be realised by using flexiblewaveguides such as optical fibres.

[0028] Thus, in a third aspect, the present invention provides anapparatus for investigating a sample is provided, comprising a source ofa beam of radiation, a source of a reference beam, an optical subsystemfor manipulating the beam between source and detector, and means fortranslating the subsystem relative to the sample, wherein the referencebeam enters the subsystem through an electromagnetic radiation guide,one end of the guide being fixed with respect to the source, the otherend of the guide being fixed relative to the subsystem.

[0029] This aspect allows for the subsystem to be translated in threedimensions whilst maintaining the path length of the reference beam asit is retained within a guide, one end of which is fixed in relation tothe radiation sources, the other end fixed relative to the subsystem.This ensures that the phase relationship between the source andreference beams entering the subsystem is maintained. Preferably, thesource is also outside the subsystem and the source beam enters thesubsystem through an electromagnetic radiation guide. Preferably, theelectromagnetic radiation guides are optical fibres. Any guide may beused which maintains the absolute path of the beams, and which maintainsthe coherence of the beams.

[0030] In apparatus according to any of the above three aspects of theinvention, the detector may be a direct detector of THz radiation or itmay be of the type which converts THz radiation into an easily readablesignal.

[0031] For example, the detector may comprise a non-linear crystal whichis configured such that upon the radiation of a probe beam and a THzbeam, the polarisation of the probe beam is rotated. The probe beam canbe of a frequency which can be easily measured (for example nearinfra-red). Typical crystals which exhibit this effect, the so-called“AC Pockels” effect are GaAs, GaSe, NH₄H₂PO₄, ADP, KH₂PO₄, KH₂ASO₄,Quartz, AlPO₄, ZnO, CdS, GaP, BaTiO₃, LiTaO₃, LiNbO₃, Te, Se, ZnTe,ZnSe, Ba₂NaNb₅O₁₅, AgAsS₃, proustite, CdSe, CdGeAs₂, AgGaSe₂, AgSbS₃,ZnS, organic crystals such as DAST (4-N-methylstilbazolium. This type ofdetection mechanism is generally referred to as ‘Electro-optic sampling’or EOS.

[0032] Alternatively, the detector could be a so-called photoconductingdetector. Here, the detector comprises a photoconductive material suchas low temperature grown GaAs, Arsenic implanted GaAs or radiationdamaged Si on Sapphire. A pair of electrodes, for example in a bow-tieconfiguration or in a transmission line configuration are provided on asurface of the photoconductive material. When the photoconductivematerial is irradiated by the reflected radiation and also, the probebeam, a current is generated between the two electrodes. The magnitudeof this photovoltage current is an indication of the magnitude of theTHz signal.

[0033] The present invention is primarily intended for use in the THzregime. Although it is possible to generate terahertz radiationdirectly, the most efficient terahertz generation can generally beachieved by converting an input beam or pump beam into a terahertz beam.Therefore, a frequency conversion member is preferably provided betweenthe source of the pump beam and the sample. This frequency conversiondevice may be situated anywhere between the source and sample and can beused to irradiate the sample with radiation of a frequency different tothat of the pump beam. It is therefore possible to irradiate the samplewith radiation frequencies which are not easily directly produced, in asimplified manner.

[0034] There are many possible options for the frequency conversionmember. For example, the frequency conversion member may comprise anon-linear member, which is configured to emit a beam of emittedradiation in response to irradiation by a pump beam. Preferably, thepump beam comprises at least two frequency components, (or two pumpbeams having different frequencies are used), the non-linear member canbe configured to emit an emitted beam having a frequency which is thedifference of the at least two frequencies of the pump beam or beams.Typical non-linear members are: GaAs or Si based semiconductors. Morepreferably, a crystalline structure is used. The following are furtherexamples of possible materials:

[0035] NH₄H₂PO₄, ADP, KH₂PO₄, KH₂ASO₄, Quartz, AlPO₄, ZnO, CdS, GaP,BaTiO₃, LiTaO₃, LlNbO₃, Te, Se, ZnTe, ZnSe, Ba₂NaNbsO₁₅, AgAsS₃,proustite, CdSe, CdGeAs₂, AgGaSe₂, AgSbS₃, ZnS, GaSe or organic crystalssuch as DAST (4-N-methylstilbazolium).

[0036] In order to produce an emitted beam having a frequency in the THzregime, preferably the at least two frequencies of the pump beam orbeams are in the near infra-red regime. Typically, frequencies between0.1×10¹² Hz and 5×10¹⁴ Hz are used.

[0037] Alternatively the frequency conversion member is aphotoconducting emitter, such an emitter comprises a photoconductivematerial such as low temperature grown or arsenic implanted GaAs orradiation damaged Si or Sapphire.

[0038] Electrodes which may be of any shape such as a dipolearrangement, a double dipole arrangement, a bow-tie arrangement ortransmission line arrangement are provided on the surface of thephotoconductive material. At least two electrodes are provided. Uponapplication of a bias between the electrodes and irradiation of a pumpbeam(s) having at least two different frequency components, a beam ofradiation is emitted having a frequency different to that of the atleast two frequency components of the pump beam or beams.

[0039] Preferably, the incident beam is pulsed beam comprising aplurality of frequencies. If a pulse having a plurality of frequenciespasses into a sample and onto a detector, the various frequencies willnot all arrive at the detector at the same time due to the frequencydependent response of the sample. Thus, a scanning delay line isinserted into the path of either the probe beam or pump beam. Byplotting the signal against the delay time (of the scanning delay line),the detected waveform can be measured.

[0040] In the above described apparatus, at least part of the apparatus,the optical subsystem, moves relative to the sample. In order to achievethis, the apparatus preferably further comprises mounting means whichare used to mount the sample. In some situations, it will beadvantageous to provide a member which is substantially transparent tothe radiation of interest, in this THz radiation for the sample to besit upon. Thus, the radiation must pass through this member in order toirradiate the sample. This member can potentially give rise to internalreflections. Therefore, there is a need to try and at least partiallyremove reflections due to the member.

[0041] In a fourth aspect, the present invention provides a method ofinvestigating a sample, a method comprising:

[0042] (a) irradiating a sample with an irradiating beam of radiation,through a member which is substantially transparent to the beam ofradiation;

[0043] (b) detecting the radiation reflected from the sample;

[0044] (c) irradiating the said member in the absence of the sample; and

[0045] (d) detecting radiation reflected from the member during step(c);

[0046] (e) subtracting the signal measured by the detector during step(d) from the signal measured by the detector during step (b).

[0047] Steps (a) and (b) can be continually repeated during a scanningoperation and the same signal measured in step (d) can be usedregardless of the current scanning position.

[0048] Steps (c) and (e) generally referred to as measuring the “baseline” signal and the above method is generally termed base linesubtraction. The base line signal can be measured before or afterscanning the sample. For example, if the system is used to measure aplurality of samples, then the base line signal only needs to bemeasured once. Generally, the above system will be used during pulsedimaging where the beam of radiation comprises a plurality offrequencies.

[0049] The method of subtracting the signal due to the window from thesample can be performed in the time or frequency domain.

[0050] In a fifth aspect, the present invention provides an apparatus,the apparatus comprising an emitter for emitting a beam of radiation toirradiate the sample; a detector for detecting radiation reflected fromthe sample; a member, which is substantially transparent to radiation,located between the sample and the emitter and detector; means forsubtracting a predetermined signal detected by the detector from afurther signal detected by the detector.

[0051] It may also be desirable to measure a reference signal and dividethe signal measured in step (b) by this reference signal. For example,this reference signal could be obtained by replacing the sample with aknown reference, for example a silver mirror.

[0052] The baseline signal can be subtracted from both the sample signaland also the reference signal. Alternatively, the baseline and referencesignals can change roles and the reference signal can be subtracted fromboth the sample signal and the baseline signal, the reference subtractedsample signal being divided by the reference subtracted baseline signal.Signals may be subtracted from one another in the time or the frequencydomain. Division of any signals should be performed in the frequencydomain

[0053] The obtained THz signal regardless of whether or not it has beensubjected to baseline subtraction or division by a reference signal ispreferably filtered.

[0054] Thus, in a sixth aspect, the present invention provides method ofinvestigating a sample, the method comprising the steps of:

[0055] (a) irradiating a sample with a beam of pulsed radiationcomprising a plurality of frequencies;

[0056] (b) measuring the beam reflected by or transmitted through thesample and obtaining a signal representative of the measured beam;

[0057] (c) multiplying the signal of step (b) in the frequency domain bythe complex Fourier transform of function F(t), wherein F(t) is anon-zero function whose integral between time limits t₁ and t₂ is zero,where t₁ and t₂ are chosen on the basis of the time delay introduced bythe sample.

[0058] To clarify, the sample will introduce a time delay into the pathof the beam. Considering the case of transmission, if the sample has thesame properties as free space, then the radiation will pass through thesample without any delay. However, if the sample (as it almost certainlywill) introduces a time delay into the beam, then t₁ will be set to anegative value which has a magnitude is equal to or larger than theexpected value of this time delay. t₂ will generally, be set to thepositive value of t₁.

[0059] Similarly, during reflection, t₁ will be set to a negative valuewhich has a magnitude is equal to or larger than the expected value ofthis time delay introduced by the pulse being reflected from the deepestpoint of interest in the sample. t₂ will generally, be set to thepositive value of t₁.

[0060] Generally, the method of the above aspect of the presentinvention will be performed using the above described apparatus wherethe radiation is measured using a reference beam and wherein a scanningdelay line is introduced into the path of the reference beam orirradiating beam in order to measure the phase change introduced by thesample. In this situation, t₁ and t₂ will be the negative and positivelimits of the scanning delay line. These may be set to the duration ofthe pulse or possibly a shorter time range.

[0061] Preferably function F(t) comprises a Gaussian componentGenerally, t₁ and t₂ are symmetric about 0, preferably F(t) is alsosymmetric about zero.

[0062] As the signal is usually digitally sampled, strictly a summationis performed as opposed to an integral.

[0063] A particularly preferable form of F(t) is provided by:${F(t)} = {\frac{2}{\pi}\left\{ {\frac{^{{- 2}{(\frac{t}{\alpha})}^{2}}}{\alpha} - \frac{^{{- 2}{(\frac{t}{\beta})}^{2}}}{\beta}} \right\}}$

[0064] where α and β are constants.

[0065] Preferable α is substantially equal to the shortest pulse lengthof the beam of pulsed radiation and β is set to be much longer than thepulse length, typically 5 to 100 times the pulse length. However, bothof these values will generally be optimised by the operator.

[0066] If β is greater than or comparable to the time which it takes theradiation to penetrate the sample to the point of interest then, F(t)can take the simplified form:${F(t)} = {\frac{2}{\pi}\left\{ {\frac{^{{- 2}{(\frac{t}{\alpha})}^{2}}}{\alpha} - \frac{1}{T}} \right\}}$

[0067] where α is a constant which is substantially equal to theshortest pulse length of the beam and T is substantially equal to thetime which it takes the beam of radiation to penetrate to the deepestpoint of interest in the sample.

[0068] Preferably data which has been baseline subtracted is filtered.Thus, preferably, step (b) comprises:

[0069] (b(i)) irradiating a sample with a beam of radiation, through amember which is substantially transparent to the beam of radiation;

[0070] (b(ii)) detecting the radiation reflected from the sample;

[0071] (b(iii)) irradiating the said member in the absence of thesample; and

[0072] (b(iv)) detecting radiation reflected from the member during step(b(iii));

[0073] (b(v)) subtracting the signal measured by the detector duringstep (b(iv)) from the signal measured by the detector during step(b(ii)).

[0074] The data has also been preferably divided by a reference signalbefore filtering. Thus, step (b) may comprise:

[0075] (b(i)) irradiating the sample with a beam of radiation, through amember which is substantially transparent to the beam of radiation;

[0076] (b(ii)) detecting radiation which is transmitted through orreflected from the sample;

[0077] (b(iii)) irradiating the said member in the presence of areference sample having known reflection and/or transmissioncharacteristics; and

[0078] (b(iv)) detecting radiation which is reflected from ortransmitted through the reference sample during step (b(iii));

[0079] (b(v)) dividing the signal measured by the detector during step(b(ii)) with the signal measured by the detector during step (b(iv)).

[0080] More preferably the signal has been baseline subtracted anddivided by a reference signal prior to filtering. Thus step (b) maycomprise:

[0081] (b(i)) irradiating the sample with a beam of radiation, through amember which is substantially transparent to the beam of radiation;

[0082] (b(ii)) detecting radiation which is transmitted through orreflected from the sample;

[0083] (b(iii)) irradiating the said member in the presence of areference sample having known reflection and/or transmissioncharacteristics; and

[0084] (b(iv)) detecting radiation which is reflected from ortransmitted through the reference sample during step (b(iii));

[0085] (b(v)) irradiating the said member in the absence of the sample;

[0086] (b(vi)) detecting radiation reflected from the member during step(b(v));

[0087] (b(vii)) subtracting the signal measured by the detector duringstep (b(vi)) from the signal measured by the detector during step(b(ii));

[0088] (b(viii)) subtracting the signal measured by the detector duringstep (b(vi)) from the signal measured by the detector during step(b(iv));

[0089] (b(ix)) dividing the signal obtained in step (b(vii) with thesignal obtained in step (b(viii)).

[0090] As explained above, the reference signal and the baseline signalare interchangeable, thus, step (b) may comprise:

[0091] (b(i)) irradiating the sample with a beam of radiation, through amember which is substantially transparent to the beam of radiation;

[0092] (b(ii)) detecting radiation which is transmitted through orreflected from the sample;

[0093] (b(iii)) irradiating the said member in the presence of areference sample haying known reflection and/or transmissioncharacteristics; and

[0094] (b(iv)) detecting radiation which is reflected from ortransmitted through the reference sample during step (b(iii));

[0095] (b(v)) irradiating the said member in the absence of the sample;

[0096] (b(vi)) detecting radiation reflected from the member during step(b(v));

[0097] (b(vii)) subtracting the signal measured by the detector duringstep (b(iv)) from the signal measured by the detector during step(b(ii));

[0098] (b(viii)) subtracting the signal measured by the detector duringstep (b(iv)) from the signal measured by the detector during step(b(vi));

[0099] (b(ix)) dividing the signal obtained in step(b(vii) with thesignal obtained in step (b(viii)).

[0100] Unwanted artefacts in the image may also be removed byappropriate choice of the thickness of the emitter or detector such thatartefacts due to internal reflection within the emitter or detectoroccur at longer delay times than those of interest in the sample.

[0101] Thus, in a seventh aspect, the present invention provides anapparatus for investigating a sample with a beam of radiation, theapparatus comprising an optically active element for irradiating thesample and/or detecting radiation reflected from the sample, theoptically active element having at least two interfaces, the distancebetween the interfaces being great enough such that the beam ofradiation takes longer to travel between the interfaces than it does totravel from the surface of the sample to the deepest point of interestwithin the sample.

[0102] The optically active element can be an emitter, detector ortransceiver.

[0103] The apparatus may comprise two optically active elements, wherethe first optically active element is configured as an emitter and thesecond as a detector, wherein the second optically active elementcomprises at least two interfaces, the distance between the interfacesbeing great enough such that the beam of radiation takes longer totravel between the interfaces than it does to travel from the surface ofthe sample to the deepest point of interest within the sample.

[0104] The optically active element may be configured as an emitterwhich emits the beam of radiation having the desired frequencies inresponse to irradiation by at least one input beam having a differentfrequency. Such an emitter may comprise an optically non-linear materialor a photo-conductive material.

[0105] The optically active element may be configured as a detectorwhich detects the beam of radiation having the desired frequencies usingat least one probe beam having a different frequency to that of the beamof radiation. Such a detector may comprise an optically non-linearmaterial which is configured to alter the polarisation of the probe beamin response to irradiation with the beam of radiation or aphotoconductive material which is configured to generate a currentwithin the photoconductive material in response to irradiation with boththe beam of radiation and the probe beam.

[0106] The present invention will now be described with reference tofollowing non-limiting embodiments, in which:

[0107]FIG. 1 is a schematic diagram of an imaging apparatus forming afirst embodiment of the present invention;

[0108]FIG. 2 is a schematic diagram of an imaging apparatus forming asecond embodiment of the present invention;

[0109]FIG. 3 is a schematic diagram showing an apparatus in accordancewith a third embodiment of the present invention;

[0110]FIG. 4 is a schematic diagram showing an apparatus in accordancewith a fourth embodiment of the present invention;

[0111]FIG. 5 is a schematic diagram showing an apparatus in accordancewith a fifth embodiment of the present invention; and

[0112]FIG. 6 is a schematic diagram showing an apparatus in accordancewith a sixth embodiment of the present invention.

[0113]FIG. 7 shows an apparatus in accordance with a farther embodimentof the present invention;

[0114]FIG. 8 shows the apparatus of FIG. 7 and its control system;

[0115]FIG. 9 is a schematic of the principle optics used to image asample through a window,

[0116]FIGS. 10a, 10 b, 10 c, show THz radiation being reflected from awindow, a reference reflector and window a sample and windowrespectively;

[0117]FIG. 11 shows data from a sample measured in accordance with anembodiment of the present invention, the THz electric field (y-axis) isplotted against the time-delay of the signal (the x-axis), the raw datais shown by a dotted line, the solid line shows the same data afterbaseline subtraction.

[0118]FIG. 12a shows a plot of electric field (y axis) in arbitraryunits against time delay (x axis) arbitrary units for a THz scan ofskin, the data has been filtered and baseline subtraction has beenperformed, FIG. 12b shows the same data without baseline subtraction;

[0119]FIG. 13 is a schematic demonstrating the multiple internalreflections which can occur within an emitter during THz reflectionimaging;

[0120]FIG. 14 shows an image of a tooth where ghost images due toreflections within the generation and detection crystals can be seen;

[0121]FIG. 15 shows the tooth of FIG. 14 measured using a thickeremitter;

[0122]FIGS. 16a and 16 b show visible images of a human incisor, FIG.16a shows the outside image whereas 16 b shows the inside side image,front and back faces are identified in FIG. 16b;

[0123]FIGS. 17a and 17 b show THz images of the from and back facesidentified in FIG. 16b; and

[0124]FIG. 18 shows a THz image of a tooth indicating the enamel air andenamel dentine interfaces.

[0125]FIG. 1 shows schematically an apparatus according to the inventionThe apparatus comprises a source 1, detector 2, a subsystem 3 containingfocusing optics 10, 11 and a substantially transparent mounting means 9for sample 7.

[0126] In this embodiment, the source 1 and detector 2 are provided onopposite sides of the subsystem 3. The source 1 provides abeam ofirradiating radiation 4 which is directed by the optics 10, 11 inoptical subsystem 3 onto sample 7. The irradiating radiation is thenreflected 5 back into subsystem 3 and the beam of reflected radiation 5is then directed into detector 2.

[0127] The subsystem 3 is translatable along a translation axis 6 andthe source beam 4 and reflected radiation beam 5 are parallel to thistranslation axis 6, as they respectively enter and exit subsystem 3.

[0128] The subsystem 3 includes a first parabolic mirror 10 whichreflects and focuses the source beam 4 to a focus 8 outside thesubsystem 3. Ideally, the focus 8 is configured such that it is at thepoint of interest of the sample 7. The subsystem also comprises secondparabolic mirror 11, which directs the beam of reflected radiation 5 toexit the subsystem 3 parallel to the translation axis 6 and intodetector 2.

[0129] Although parabolic mirrors are used as the optics which directradiation to and from the sample 7, it is possible to use other opticswhich will focus and/or reflect the source and reflected beams 4, 5 inthe required manner.

[0130] The sample 7 is placed on mounting means 9 which remain fixedwith respect to both the source 1 and the detector 2. The mounting meansin this particular example is a fixed window which allows thetransmission of the beam of irradiating radiation 4 and the reflectedradiation 5. The sample is provided on the opposing side of the window 9to the optical subsystem 3.

[0131] As the subsystem 3 moves along the translation axis 6 it causesthe focus 8 in the beam 4, 5 to be scanned across the sample 7.

[0132] The above apparatus ensures that the path length travelled by thebeam 4, 5 remains constant, regardless of the position of the subsystem3 along the translation axis 6. Thus phase of the beam 4, 5 is notaffected by the movement of the subsystem 3 along the x translation axis6 in the absence of the sample.

[0133] In order to derive sensible information from the sample 7, thereis a need to measure the change in the phase of the source beam 4 causedby the sample 7. Thus, detector 2 needs to know some information aboutthe phase of the radiation leaving the source 1 The dotted lineindicates that such information is required. A way of doing this is touse a reference beam (not shown) which has a phase related to that ofthe source beam 4 and which is received by the detector. Actually manydetectors of THz radiation require a reference beam in order to detectradiation received by the detector. Such detectors will be described inmore detail later.

[0134] In the apparatus of FIG. 1 the path length between the source andthe detector remains the same, thus a reference beam having a known pathlength can be provided to the detector 2. The use of a reference beamwill be described in great detail with reference to FIG. 3.

[0135]FIG. 2 shows, schematically, a second embodiment of the invention.However, in this case, two sources of radiation 101, 114 are provided,one for the source beam 104 and the other 114 for the reference or probebeam 104 for the detector 102. The detector 102 is provided within theoptical subsystem 103 and moves with the optical subsystem. Theapparatus is configured such that movement of the detector 102 causesthe same change in the path length of the source beam and the referencebeam, thus preserving the phase relationship between the two beams.

[0136] A first source 101 emits a source beam 104 and a detector 102detects the beam of reflected radiation 105 after it has been reflectedfrom sample 107. A subsystem 103 is movable along translation axis 106.The source beam 104 is parallel to the translation axis 106 when itenters the subsystem 103. The detector 102 is placed within thesubsystem 103. A reference source 121 provides a reference beam 114which enters the subsystem 103 parallel to both the source beam 104 andtranslation axis of the subsystem 103 and is also detected by thedetector 102.

[0137] The subsystem 103 includes a parabolic mirror 110 which reflectsand focuses the source beam 104 to a focus 108 outside the subsystem103. The subsystem 103 also includes a further parabolic mirror 111,which collects reflected radiation from the sample 107 and directs it tothe detector 102. Other optical elements could be used instead ofparabolic mirrors.

[0138] Sample 107 is placed on mounting means 109, which remains fixedwith respect to both the source 101 and the reference source 121. As thesubsystem 103 moves along the translation direction 106 the focus 108 ofthe source beam 104 is scanned across the sample 107.

[0139] The apparatus of FIG. 2 ensures that the phase difference betweenthe source beam 104 and the reference beam 114 remains constant as theyenter the subsystem regardless of the movement of the subsystem 103along the translation axis 106. This is because any change in the pathlength of the source beam 104 from the source to its entry to thesubsystem 103 due to the movement of the subsystem 103 along thetranslation axis 106 is the same as the change in the path length of thereference beam 114 from the reference source 121 to its entry to thesubsystem 103.

[0140] In both the above embodiments, the source 1, 101 is a source ofcoherent radiation, for example a laser, and more particularly a modelocked Ti:Sapphire laser may be used to produce near infrared radiation.The apparatus of the present invention is primarily intended for usewith THz radiation. Although dedicated sources of THz radiation exist,generally, THz radiation is produce using a frequency conversion member,such as a non-linear crystal which is configured to generate THzradiation in response to irradiation by an input beam having a frequencydifferent to that of the THz radiation or a photoconductive materialwhich is configured to emit THz radiation upon application of a bias andirradiation by a input beam having a different frequency to that of theTHz radiation.

[0141] The source beam may be used directly or it may be passed throughsuch a frequency conversion member. The part of the source beam which isincident on the frequency conversion member will be referred to as thepump beam and the part of the source beam emitted from the frequencyconversion member will be referred to as the THz beam. Although thisinvention is primarily intended for use in the THz regime, otherfrequencies could also be used.

[0142] The frequency conversion member may be driven by using a pumpbeam comprising pulsed infra-red radiation which excites the member toproduce a single cycle terahertz pulse centred at around 1 THz, thespectrum being broad due to the short pulse length, while maintainingthe phase characteristics from the near infared pulse. The frequencyconversion member may be placed in the path of the source beam eitherinside the optical subsystem or outside the optical subsystem 1, 103.

[0143] The parabolic mirror 10, 110 used to focus the source beam 4, 104is used to a small focus 8, 108, typically less than from 1 mm whileensuring that radiation from across the whole cross section of the beam4, 104 arrives at the focus 8, 108 substantially simultaneously. Asimple off axis parabolic mirror may be used for near infrared radiationor terahertz radiation. No specialised optical equipment is required forthe manipulation of the terahertz pulses i.e. optical equipment which issuitable for infrared radiation manipulation may be used

[0144] The detector 2, 102 may be any detector suitable for detectingthe frequency of radiation supplied by the source 1, 101 and anyreference source 121. For example, in the case where the source 1, 101is a terahertz frequency beam having a range of frequencies centredaround 1 THz, and a reference source 121 is supplied and is a nearinfrared beam, for example at a frequency of 1×10¹⁴ Hz, the detector maybe either a non-linear crystal or a photo-conducting device. These aredescribed in greater detail in the embodiments relating to FIGS. 3 and 5below.

[0145] In the embodiment of FIG. 2, the reference source 121 is a sourceof coherent radiation, but is not necessarily of the same frequency asthe source 101. A common source can be used for the source beam 104 andreference beam so that they are in phase.

[0146] A beam splitter (discussed in further embodiments) such as asemi-transparent mirror, or a prism, is used to split the beam toproduce the source beam 104 and reference beam 114.

[0147] The mounting means may be made out of any material substantiallytransparent to the radiation being used at the focus 8, 108. In the caseof a terahertz frequency apparatus, the mounting means is a crystalquartz window. It is possible that the mounting means 9, 109 has a holethrough it such that radiation can reach the sample 7, 107 from theoptical subsystem 3, 103.

[0148] The movement of the subsystem 3, 103 may be by any conventionalmeans (not shown). For example, a stepper motor and/or rack & pinionsystem (not shown) may be employed. This allows accuracy of displacementand enables the position of the subsystem 3, 103 to be known by acontrolling system (not shown). Thus the position of the focus 8, 108can be determined with a high degree of accuracy. Typically, an accuracyof position of 0.01 mm is sufficient for the subsystem 3, 103, whenterahertz radiation is used to irradiate the sample 7, 107.

[0149] A third embodiment of the present invention will now be describedwith reference to FIG. 3. The apparatus of FIG. 3 allows a sample to bescanned in two orthogonal directions. For the purposes of this example,the two directions will be the x direction and y direction. Theapparatus uses a x optical subsystem (similar to that described withreference to FIG. 1) placed within a y optical subsystem (similar tothat described with reference to FIG. 2).

[0150] Source 221 comprises a mode locked laser generating sub 200 fspulses in the near infrared spectral range (approx. 1×10¹⁴ Hz).Specifically, a Cr:LiSAF laser (100 MHz, 100 fs and 100 mW) is used.Other sources can be used, for example, a mode locked Ti:Sapphire laser.These pulses of near infrared radiation are split using a beam splitter240, into a pump pulse 224, and a reference pulse or probe pulse, 214.

[0151] The pump pulse 224 travels through scanning optical delay line240, which introduces an oscillating delay into the pump pulse 224. Thescanning delay line comprises a retro reflection mirror 242 whichreflects the pump beam back on itself and onto planar mirror 243. Mirror243 is configured to reflect the beam out of the scanning delay line 240at right angles to itself and into the y optical subsystem 213 parallelto the translation axis. By moving the retroreflector 242 back and forthparallel to the direction on the pump beam, the optical path length isoscillated.

[0152] The irradiating radiation 224 then enters the y subsystem 213parallel to the translation direction 216 of the y subsystem 213. Onentering the y subsystem 213, the pump beam 224 is reflected through 90°by mirror 236 a and then through a further 90° by mirror 236 b such thatthe path of the pump beam 224 is parallel to the direction which itentered subsystem 213. Mirrors 236 a and 236 b serve to direct the pumpbeam 224 onto frequency conversion member 201 which converts the nearinfrared pump beam 224 into a beam of THz radiation 204.

[0153] Frequency conversion member 201 may comprise a non-linearcrystal. If a non-linear crystal is irradiated by two differentfrequencies ω₁ and ω₂, radiation having a frequency which is thedifference or the sum of these frequencies is outputted. Preferably, thepump beam and crystal is chosen such that the crystal outputs radiationhaving a frequency which is the difference of the two frequenciesimpinging on the crystal.

[0154] As the frequency conversion member 201 is irradiated withinfrared radiation ω₁ and ω₂, the electrons vibrate to emit radiationwith a THz frequency, the THz radiation ω_(THz)=ω₁−ω₂.

[0155] Often, such a frequency conversion member 201 will have phasematching means in order to keep the transmitted THz signal and theincident radiation in phase as they pass through the frequencyconversion member 201. (For example, see GB 2 343 964. Such phasematching can be achieved by providing the frequency conversion member201 with a variation in its refractive index configured to keep the twosignals in phase (at all points) as they pass through the frequencyconversion member 201.

[0156] The frequency conversion member 201 may also be a so-calledphotoconducting emitter comprising a photoconducting material such aslow temperature grown GaAs or radiation damaged Si on Sapphire. A pairof electrodes are formed on a surface of the photoconducting material.THz radiation will be emitted having a frequency (or frequencies) whichis the difference in the frequencies of the photoconducting materialwith the pump beam and upon application of a bias between the twoelectrodes. (For further details on photoconductive emitters see U.S.Pat. No. 5,729,017).

[0157] The terahertz beam pulse 204, produced by the frequencyconversion member 201, is collimated using one or more parabolic mirrors230, which are suitable for use with terahertz frequency radiation.Parabolic mirror 230 serves to direct the radiation parallel to the xtranslation axis 206 and to maintain the phase of the THz radiationacross its cross section. The incident THz pulse 204 then enters the xsubsystem 203 collimated and parallel to the x translation axis 206.

[0158] The x subsystem 203 performs in a similar manner to thatdescribed in the first embodiment and comprises a set of elements formanipulating the pulse of radiation 204, 205 between its to entry andexit from the x subsystem 203. The x subsystem can move along thex-translation axis.

[0159] The x subsystem 203 comprises a first parabolic mirror 210, whichfocuses the THz beam 204 to a focus 208 outside the x and y subsystems203, 213.

[0160] The sample (not shown) is placed on mounting means (not shown)which remain fixed along the x translation axis 206 with respect to boththe frequency conversion device 201 and the detector 202, and fixed inthe y translation axis 216 with respect to the source 221 such that thebeam can be scanned along the x and y axes of the sample.

[0161] The pulse 204 is incident on the sample (not shown) in adirection largely orthogonal to the x and y translation axes 206, 216.However, it is possible to arrange the pulse 204 to be incident on thesample 207 at any angle. Radiation reflected by the sample is collectedby y parabolic mirror 211 and is collimated parallel to the xtranslation axis 206 of the subsystem 203. The reflected THz radiation205 exits the x subsystem 203 in the same direction as the incidentpulse 204 and on the opposite side of the x subsystem 203.

[0162] The reflected THz radiation 205 which exits the x subsystem 203back to the y subsystem where it is collected by parabolic mirror 231and directed onto detector 202. Probe beam 214 is also incident ondetector 202.

[0163] Probe beam 214 from beam splitter 220 is directed though fixeddelay line 241 and into the y subsystem 213. The probe beam 214 and THzbeam 205 need to reach the detector 202 in phase with one another. Ascanning delay line 240 is placed in the path of the pump beam 224 inorder to scan the phase of the pump beam in order to measure the phasechanges caused by the sample. A fixed delay 241 is also included in thereference beam 214 in order to ensure that the probe and reflected THzbeams can be brought into phase at detector 202. In order to allowtranslation of the y subsystem 213 along a y translation axis 216, thepulses 214, 224 enter the y subsystem 213 parallel to this y translationaxis 216, and preferably are collimated.

[0164] The difference in path length between the probe 214 and pump 224beams remains the same regardless of the position of the y subsystem 213along the y translation axis 216 so that the phase relationship betweenthe probe and pulse beams is maintained.

[0165] Once the probe beam 214 enters the y subsystem, it is reflectedthrough 90° by first planar mirror 235 a and it is then reflected thougha further 90° by second planar mirror 235 b such that the probe beam 214is still travelling parallel to the y translation axis. Mirror 235 bserves to direct the probe beam through a hole in parabolic mirror 231such that the probe beam can be combined with the radiation reflectedfrom the sample for detection.

[0166] As the parabolic mirror 230 is arranged so that the terahertzpulse 204 enters the x subsystem 203 collimated and parallel to the xtranslation axis 206, and parabolic mirror 211 causes the pulse to becollimated and parallel to the x translation axis as it leaves the xsubsystem 203, this allows the x subsystem 203 to be moved along the xtranslation axis without varying the total path length travelled by theterahertz radiation 204, and reflected/transmitted radiation 205. Henceno phase change in the pulse is introduced at the detector 202 by virtueof the movement of the x subsystem 203 along the x translation axis 206.

[0167] The y subsystem 213 is translatable along the y translation axis216, carrying the x subsystem 203 with it. The x subsystem 203 is withinthe y subsystem 213 so that the x subsystem 203 may still be translatedalong the x translation axis 206, while the y subsection 213 istranslated along the y translation axis 213.

[0168] The detector is of the photoconducting type, comprising aphotoconductive detection member which may be, for example, GaAs,radiation damaged Si on Sapphire etc. The THz radiation is incident on afirst surface of the photoconductor 202. A pair of electrodes (notshown) are located on the photoconductor 202 on the first surface.

[0169] The probe beam 214 illuminates the surface of the detectorbetween the electrodes. The reflected Terahertz radiation 205 which iscollected by the lens induces a photocurrent through the region betweenthe electrodes which is being illuminated by the probe beam 214. Thecurrent, which can be detected by the electrodes, is proportional to thestrength of the THz field. The current is generated when the THzradiation arrives at the detector 202 in phase with the probe beam.

[0170] The travel of the terahertz radiation pulse in the sample (notshown) will induce a delay in the pulse. This delay will depend upon howfar into the sample 207 the pulse travels before being reflected, orotherwise directed back to the parabolic mirror 211. It will also dependon the refractive index of the sample at each frequency within thepulse. The pulse 205 is therefore broken up into different temporalelements by travelling through the sample, these different elementsarriving at the detector at different times. These elements are comparedwith the reference pulse.

[0171] However, all frequencies of the reference pulse arrive at thesame time. In order to overcome this, the scanning time delay is used tointroduce a time delay in either the pump pulse or the probe pulse, (inthis case, the pump pulse), so that it can be compared to parts of thepump pulse with different time delays. Therefore, it is possible todetect the delay introduced due to the sample at different frequencies,and the extent of the delay will give an indication of how far into asample the pulse travelled at that frequency.

[0172] Also, because of the different reflectivities and absorptioncharacteristics for different frequencies of terahertz radiation for agiven sample, an indication of the material contained within a sample atvarious depths can be obtained. Since reflection of radiation will occurat a boundary between two materials with different opticalcharacteristics, an indication of the depth or thickness of, forexample, a surface layer may be determined.

[0173] Additionally, when the y subsystem 213 is translated along the ytranslation axis 216, the change in path length of each of the pump 224and probe 214 pulses is the same. Although the probe beam 214 does nottravel the along the same optical path as the pump pulse for irradiatingthe sample, any changes to the sample path length are kept the same asthose to the probe path length by the arrangement so that a comparisonof the phase of the two is possible, the only phase change which will beintroduced between the two pulses will be due to the sample 207.

[0174] The y translation axis 216 is orthogonal to the x translationaxis 206 so that independent movement in two dimensions of an orthogonalco-ordinate system is possible. Although the terms ‘x’ axis and ‘y’ axishave been used, the subsystems could be configured to move along any twoaxis regardless of whether or not they are orthogonal or non-orthogonal.

[0175] Although the above description has discussed reflecting radiationfrom the sample, to image the sample. In such an arrangement, the sample(not shown) is placed off axis from the parabolic mirrors 210, 211.However, transmission measurements are also possible if the sample isplaced between the parabolic mirrors 210, 211. Other focusing elementscould be used in addition to or instead of parabolic mirrors.

[0176]FIG. 4, shows schematically a fourth embodiment which is similarto the third embodiment. However, in this embodiment, the reflectedradiation and the probe pulse are incident on opposing sides of thedetector 202.

[0177] To avoid unnecessary repetition, like reference numerals will beused to denote like features. Probe pulse 214 enters the y subsystemparallel to the y translation axis and is reflected using mirrors 235 aand 235 b which direct the probe beam 214 parallel to the y translationaxis. The probe beam then impinges on planar mirror 237 a which rotatesthe probe beam through 90° and onto mirror 237 b. Mirror 237 b reflectsthe beam through a further 90° such that the beam is reflected backparallel to its original path and onto the front of detector 202.

[0178] The path of the pump beam and reflected THz beam 205 remainsidentical to that previously described with reference to FIG. 3.

[0179] The reflected THz radiation 205 from the sample is incident onthe back surface of the detector 202 and is collected by lens 242, whichmay be hemispherical or have another shape. The lens is provided on theback surface of detector 202.

[0180] On the opposing side of the detector 202 a pair of electrodes(not shown) is located. The probe beam 214 is incident on this side ofthe detector 202 to the side of the detector which receives thereflected THz beam 205.

[0181] Alternatively, they may be triangular and arranged in the shapeof a bow-tie to from a so-called bow-tie antenna. They may also beinterdigitated electrodes at the centre of a bow-tie or spiral antenna.A transmission line arrangement of electrodes such as that disclosed inFattinger et al Appl. Phys. Lett. 54 4901 (1989) may also be used.

[0182]FIG. 5 shows schematically a fifth embodiment of the invention.This embodiment functions in largely the same manner as the embodimentof FIG. 3 However, in FIG. 5, the detector 202 is replaced by anon-linear crystal which is configured to operate as an electro-opticsampling (EOS) mixing crystal. To avoid unnecessary repetition, likereference numerals have been used to denote like features.

[0183] In this embodiment, the probe pulse 214 and the reflectedterahertz pulse from the sample (not shown) are both incident on thedetector 202 on the same side in the same manner as described withreference to FIG. 3. The EOS mixing crystal 202 is not the finaldetection means, but instead the EOS mixing crystal encodes informationfrom terahertz pulse 205 onto the probe pulse 214 by way of altering thepolarising of the probe pulse 214 as it passes through the EOS mixingcrystal 202. In order for this to happen, the probe pulse 214 mustarrive at the detector 202 in phase path the reflected THz radiation205.

[0184] An EOS mixing crystal 202 utilises the physical phenomenon knownas the AC Pockels effect. The transmitted THz radiation 205 from thesample (not shown) is detected by passing the probe beam 214 through thecrystal 202 with the reflected THz pulse 205. The reflected THz pulse205 modulates the birefringence of the detection crystal by the ACPockels effect.

[0185] Prior to entry into the crystal 202, the THz pulse 205 and theprobe pulse 214 are polarised. Where there is no THz pulse 205, theprobe pulse 214 passes unaffected through the EOS mixing crystal 202.When the probe pulse 214 exits the mixing crystal, it is passed into apolarisation detection system 232.

[0186] The details of the polarisation detection system are not shown asthey well known, see for example GB 2 343 964.

[0187] Since the relative phase information is contained in the one beamexiting the EOS mixing crystal 202, it is not important to have anyparticular distance from the detector 202 to the polarisation detector232, as the reference 214 and the reflected terahertz pulse 205 pulsesare now encoded in the same polarised pulse 234. Thus, the polarisationsensitive detection mechanism can be placed outside the x and ysubsystems as shown in FIG. 6.

[0188]FIG. 6 shows schematically a sixth embodiment of the invention.This embodiment is largely the same as the embodiment shown in FIG. 5,except that the polarisation detector 232 is placed outside the ysubsystem 213 rather than inside the y subsystem 213. Here, thepolarisation detection system is provided on the opposite side of the ysubsystem 213 to the to the source 221. The polarisation detector 232 isfixed with respect to the source 221.

[0189] The polarised pulse 234 is manipulated using mirrors 238 so thatit is parallel to the y translation axis 216 as it exits the y subsystem213. This means that the path length of the polarised pulse 234 does notchange as the y subsystem 213 moves along the y translation axis 216.

[0190] While the apparatus shown in FIGS. 3 to 6 have given examples ofa system which can scan the beam in two dimensions, it is readilyapparent that these techniques could be extended to three dimensionsusing the same principles as given above.

[0191]FIGS. 7 and 8 show schematically a seventh embodiment of theinvention. This embodiment includes a subsystem 53, within which anemitter 51 and detector 52 are arranged.

[0192] The emitter 51 directs the radiation 54 towards a first parabolicmirror 60 which reflects the radiation to sample 57 under investigation.The radiation 55 reflected from or transmitted through the sample 57 isreflected by a second parabolic mirror 61 to the detector 52. The firstand second parabolic mirrors 60, 61 are both within the subsystem 53(such that they move with subsystem 53) and are fixed relative to theemitter 51 and detector 52.

[0193] The emitter 51 comprises a frequency conversion member whichemits radiation having the desired frequency, in this example THzradiation, in response to irradiation by an pump beam having a differentfrequency. In this example, the pump beam is supplied to the emitterusing fibre optic cable 62.

[0194] The detector operates using a probe beam. The probe beam has aknown phase relationship to that of the pump beam. The reference beam issupplied to the detector using fibre optic cable 63.

[0195] As the subsystem 53 moves to scan along any axis, the emitter 51and the detector 52 move with the subsystem 53. The path length ofradiation travelling from the emitter 51 to the detector 52 does notchange. Neither does the path length of the probe pulse or pump pulse asthese are supplied via flexible fibre optic cables 62, 63 which can movewith the subsystem 53.

[0196]FIG. 8 shows the embodiment of FIG. 7 in more detail. Here, thesame source 71 is used as in the third and subsequent embodiments, i.e.a Cr:LiSAF laser. As described in the third and subsequent embodiments,the source 71 produces a pulse which is split into a pump beam 65 andprobe beam 64 by beam splitter 72.

[0197] The emitter may be a direct emitter of THz radiation which doesnot require a pump pulse. However in this example, the reference pulseneeds to still carry information about the phase of the radiationemitter by the emitter to the detector.

[0198] Scanning delay line 73 is provided to cause an oscillating delayin the probe pulse 64. The scanning delay line 73 is connected tomicroprocessor 80 such that the microprocessor knows the position of thedelay line at a point in time in order to analyse the detectedradiation.

[0199] The pump pulse optic fibre 62 extends between the beam splitter72 and the frequency conversion device 51. The probe pulse fibre 63extends between a delay line 73 (from the beam splitter 72) and thedetector 52. The each optic fibre 62, 63 is fixed at one end relative tothe beam splitter 72, and at the other end relative to the subsystem 53.

[0200] The emitter 51 is a photoconductive emitter which emits radiationof the desired frequency in response to irradiation with the pump beamand upon application of a bias across the electrodes. The bias voltageis applied by antenna bias control voltage system 81. The control system81 is connected to the THz emitter via electrical cable 83.

[0201] The THz detector 52 generates a current in response toirradiation by THz radiation in the presence of the probe beam. Themeasured current is carried via cable 85 to signal detection anddigitisation apparatus 87. The signal is then sent to microprocessor 80.

[0202] The subsystem 53 can be moved in-any direction relative to thesample 57 because the path length of both source 65 and reference 64beams are kept constant. This occurs because the optic fibres 62, 63 areof fixed length and are fixed at each end. The relative phase betweenthe source 65 and reference 64 beams is therefore maintained within thesystem, so that any path difference introduced is caused by the sample57. This gives the advantage that more complicated optics can be usedwithin the subsystem 53 than parabolic mirrors 60, 61, as they do nothave to collimate radiation entering and leaving the subsystem 53, andcan be specific to the configuration within the subsystem 53. Thesubsystem 53 is movable relative to the laser source 71, and thesubsystem 53 may be scanned in three dimensions with no extrasubsystems.

[0203] The data received by the detector can be analysed to find thedepth to which the sample pulse travels in to the sample. From this, apartial sample cross-section can be made up by scanning the sample pulsefocal point along the translation axis. The delay time of the opticalline can be plotted against scanning position.

[0204] The embodiments described above may be used in a wide range ofimaging applications. As terahertz radiation has no shown detrimentaleffect on animal or human tissue, it is possible to use it in a widevariety of imaging techniques on various body and other parts, bothafter removal and in situ. For example, it is possible to obtain resultsfor the thickness of the enamel on a tooth, whether from a human, orother animal. It is also possible to image layers of the skin and othertissues, including the stratum corneum and epidermis.

[0205] Many other imaging techniques are possible where the sample is atleast partially transparent to terahertz radiation, and surface mappingis possible where the sample is substantially opaque to terahertzradiation.

[0206] In FIG. 9, sample 1001 which is to be imaged is provided adjacentand in contact with window 1003. Window 1003 is positioned such thatimaging radiation can only reach sample through window 1003 andradiation reflected from sample 1001 also has to pass through window1003. The sample imaged using radiation generated by THz emitter 1005and the radiation reflected from the sample 1001 is detected using THzdetector 1007.

[0207] Radiation emitted from emitter 1005 is directed to the sampleusing parabolic mirror assembly 1009. Parabolic mirror assembly 1009also directs radiation reflected from sample 1001 into THz detector1007.

[0208] THz beams 1011 a and 1011 b are emitted from THz emitter 1005.Both of these beams 1011 a and 1011 b are directed through window 1003onto sample 1001 and reflected back onto parabolic mirror assembly 1009and into THz detector 1007. However, some of the beams which exit mirrorassembly 1009 will not pass through window 1003. Instead, some will bereflected off the underside 1013 of window 1003. The beams 1015reflected off the underside of window 1013 will also be detected by THzdetector 1007. As these beams have not been reflected from sample 1001,these beams contain no information about the sample and should beeliminated.

[0209] This problem of extraneous reflections are encountered in manysystems as windows are generally desirable. For example, a window may beprovided on a sealed case which encloses the imaging system such thatatmospheric gases/vapours are excluded from the system. A window canalso be used to define the plane of a “soft” sample (e.g. living tissue)or, to protect the THz system from external contamination.

[0210]FIG. 10 illustrates some steps which can be used in order toremove reflections due to the window from the eventual image, so calledbaseline subtraction.

[0211] In FIG. 10a, an image from the THz window 1003 is measured in theabsence of sample 1001. Initially, baseline subtraction will bedescribed using just the arrangements of FIGS. 10a and 10 c. Then, theuse of a reference beam as shown in FIG. 10b will also be explained.

[0212] In this specific example, the time domain waveform of the THzreflected from just window 1003 as shown in FIG. 10a is measured. Thetotal reflected THz radiation (reflected from both the sample 1001 andthe window 1003) is measured as shown in FIG. 10c.

[0213] The baseline signal, that measured in FIG. 10a, is denoted byB(t), in the time domain (where t represents time). A complex Fouriertransform is then applied to the signal B(t), to obtain the baselinespectrum B′(v) where (v) represents frequency. The time domain waveformmeasured with the sample of interest measured in FIG. 10c is denoted byS(t). The complex for Fourier transform of this signal is then taken toobtain the sample spectrum S′(v).

[0214] The baseline subtracted waveform in the frequency domain isobtained using the following equation:

S′(ν)−B′(ν)

[0215] This is then complex Fourier transformed in order to obtain thebaseline subtracted waveform or in the time domain.

[0216] Alternatively, baseline subtraction can be performed in the timedomain.

[0217]FIG. 11 shows a plot of measured THz electric field in arbitraryunits against the delay time in arbitrary units with baselinesubtraction (shown as a solid line) and without baseline subtraction(shown as a dotted line). The sample image here was a human finger tip.It was imaged using THz radiation having a frequency range of 20 GHz-2.5THz

[0218] In general, the image will be further defined by using areference signal and optionally, a filter function. FIG. 10b shows theuse of a reference reflector 1014. The reference reflector 1014 isprovided on the opposing side of the window 1003 to the emitter anddetector, in the same position as the sample 1001. The referencereflector 1014 is used for spectral comparison. Typically, the referencebeam will be obtained using a mirror or other sample with known andconstant reflection characteristics.

[0219] The time-domain waveform measured using the reference sample isgenerally represented by R(t). A complex Fourier transform is thenapplied to this time-domain waveform to obtain the reference spectrumR′(v).

[0220] The baseline waveform in the frequency domain is subtracted fromboth the sample waveform on the reference waveform. The baselinesubtracted sample waveform is then divided by the baseline subtractedreference waveform in the frequency in accordance with the followingequation:$\frac{{S^{\prime}(v)} - {B^{\prime}(v)}}{{R^{\prime}(v)} - {B^{\prime}(v)}}$

[0221] The above result is then complex Fourier transformed in order toobtain the final time-domain waveform.

[0222] The same reference and baseline waveform can be used as the beamis scanned across the sample as the contribution from the window shouldnot change.

[0223] Also, the above procedure could be used where it is necessary tosubtract all reflections from other optical surfaces which are presentin the sub-system.

[0224] The reference and baseline measurements can also be reversed suchthat the reference waveform is measured from just the window and thebaseline is measured with the reference reflector in place.

[0225] In general, a filter function will also be used F(t) and iscomplex Fourier transform will be represented by a F′(ν).

[0226] A typically preferred filter function is used because the THzpulse system can generate and detect pulses comprising frequencies oversome finite range, typically from less than 100 GHz to over 3 THz. Thereis a high frequency limit above which the THz signal falls below thenoise level of the detection system.

[0227] Similarly, the THz signal level falls below the noise level atlow frequencies. Thus, there is a need to remove the high and lowfrequency noise. A particularly preferable function for achieving thisis:${F(t)} = {{\frac{2}{\pi}\left\{ {\frac{^{{- 2}{(\frac{t}{\alpha})}^{2}}}{\alpha} - \frac{^{{- 2}{(\frac{t}{\beta})}^{2}}}{\beta}} \right\}}}$

[0228] The parameters α and β are selected to control the high and lowfrequency roll-off of the function. α is set approximately the shortestTHz pulse length (half cycle) obtainable within the THz system. β is setto be much longer than the THz pulse. In operation, the two parametersare optimised manually by the operator to obtain the best compromisebetween bandwidth and noise.

[0229] As the above function comprises two Gaussian functions withsimilar areas but opposite signs, the above function ensures that theintegral of the filter function for all time is zero.

[0230] If the value of β is comparable to or greater than the totaltime-delay scan range, then an alternative function can be used:${F(t)} = {{\frac{2}{\pi}\left\{ {\frac{^{{- 2}{(\frac{t}{\alpha})}^{2}}}{\alpha} - \frac{1}{T}} \right\}}}$

[0231] where T represents the total range of delay times used i.e.T=T_(max)−T_(min). This ensures that the overall integral from T_(min)to T_(max) is always zero.

[0232]FIG. 12a shows a reflection from skin where the THz electric fieldis plotted against the time delay in arbitrary units. The data shown inFIG. 12a has been both baseline subtracted, and filtered. The referencebeam has also been used.

[0233]FIG. 12b shows the same data as FIG. 12a without the baselinesubtraction but with the filter function and reference.

[0234] It is also possible to remove or at least distinguish artefactsdue to unwanted reflections by careful choice of the emitter and/ordetector thicknesses. In FIG. 13, sample 1001 has a first interface 1017a first internal interface 1019 and a second internal interface 1021. Itis required to find out the position of these three interfaces. However,there is no need to probe the sample deeper than interface 1021. The THzwaveform is generated within emitter 1005. In this simplified example,the emitter comprises a single crystal which has a first interface 1023and a second interface 1025 within the path of the beam. The first andsecond interfaces 1023 and 1025 are both capable of reflecting the THzradiation. In the ideal case, the THz radiation is generated within theemitter 1005, it follows path 1027 and is reflected off the furthestinterface 1021 (amongst other interfaces on sample 1001). The reflectedTHz signal is then directed into THz detector 1007 for detection. TheTHz detector 1007 may be a so-called photo-conducting detector where THzradiation 1027 in combination with a probe beam 1029 cause a current tobe generated in emitter 1007. Alternatively, it could be a so-called EOSdetector where the THz beam 1027 serves to rotate the polarisation ofthe probe beam 1029. By measuring the change in rotation of thepolarisation probe beam 1029, it is possible to measure the THzradiation.

[0235] The above is a rather over-simplified example where reflectionsonly occur from the interfaces 1017, 1019 and 1021 in the sample.However, typically, reflections will also occur within the emitter 1005and detector 1007. In FIG. 13, some of the THz radiation impinging onthe second interface 1025 of emitter 1005 is transmitted through theinterface towards the sample 1001. However, some of this radiation isreflected back from interface 1025 towards first interface 1023 and thenis doubly reflected from interface 1023 through interface 1025 to thesample.

[0236] It is desirable to remove the secondary reflection. In thisexample, the time which the THz radiation takes to be reflected from thefirst interface 1023 to the second interface 1025 following the path ofthe THz beam, is longer than the time which it takes the THz radiationto enter sample 1001 through interface 1017 and to reflect frominterface 1021.

[0237] Thus, the generation crystal is thick enough so that THz pulsesreflected off the back of the crystal is measured at optical delay linesgreater than that of the sample structure of interest. In FIG. 13, theTHz beam radiation travels a distance D through sample 1001 in order toreach interface 1021. On entering the sample, the beam 1007 firsttravels through the first region which has a refractive index N₁ for adistance D₁ and then travels through the second region having arefractive index of N₂ for a distance d₂. The distance which the THzbeam travels from the first interface 023 to the second interface 1025of the emitter is t. The refractive index of the emitter is ne. Forclear observation of the reflection from interface 1021, the followingequation must used: $t = \frac{{N_{1}D_{1}} + {N_{2}D_{2}}}{n_{e}}$

[0238] Similarly, the pulse width t_(d) through the detector having arefractive index of n_(d) can be calculated in the same manner. Theemitter and/or detector may have multiple interfaces. In order to beable to clearly identify the signal due to reflections from each ofthese multiple interfaces, it is desirable for each of these multipleinterfaces to be positioned such that the THz radiation takes longer totravel between two adjacent interfaces than it does to travel throughthe sample to the interface of interest 1021.

[0239]FIG. 15 shows an image of a human tooth which has been measured toa depth of about 1 mm. The generation and detection crystals need to beat least this thickness (in this case, the detection crystal is an ZnTeEOS crystal having a refractive index of 2.6 which is similar to that ofthe refractive index of dental enamel). The GaAs emitter has arefractive index of 3.6 which is slightly more than that of toothenamel. Thus, a slightly thinner crystal can be used.

[0240] To obtain the shown image, a 0.5 mm emitter crystal was used anda 1 mm detection crystal. In the shown image, the delay time is shownagainst scanning position along a line along the tooth surface. Theamplitude of the signal measured for the delay time is shown on a greyscale where white indicates a strong positive THz field. The signal fromthe top surface of the sample is shown for low delay times. A ghostimage due to reflection within the emitter is shown at a delay time ofabout 7. Also, a further ghost image which probably arises due tomultiple internal reflections within the sample 1001 is also shown.

[0241]FIG. 15 shows the same sample as FIG. 14 measured with an emittercrystal having a thickness of 1 mm. The signal due to the internal totalreflection has disappeared (while the true structural feature of thesample remains).

[0242]FIGS. 16a and 16 b show visible images of a human tooth. Theincisor has been cut in two, FIG. 16a shows an image of the outside ofthe tooth, whereas FIG. 16b shows an image of the inside of the tooth, aback-side (top rectangle) and a front side (lower rectangle) areidentified on FIG. 16b.

[0243]FIGS. 17a and 17 b show THz images of the back and front sides ofthe incisor identified in FIG. 16b.

[0244]FIG. 18 shows the THz image of the front side of the tooth of FIG.16b. A reflection artefact due to a 1 mm thick emitter crystal can beseen at the top of the figure. The air/enamel and dentine/air interfacescan also be seen

[0245] While above embodiments have made use of pulsed radiation, itwill be readily appreciated by one skilled in the art that continuousbeams of radiation could also be used, in part or in whole to replacethe pulses, a pulse being a beam of short duration comprising multiplefrequencies.

[0246] The present invention has been described above purely by way ofexample, and modifications can be made within the spirit of theinvention. The invention also consists in any individual featuresdescribed or implicit herein or shown or implicit in the drawings or anycombination of any such features or any generalisation of any suchfeatures or combination.

[0247] Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise”, “comprising”, and thelike, are to be construed in an inclusive as opposed to an exclusive orexhaustive sense; that is to say, in the sense of “including, but notlimited to”.

1. An apparatus for investigating a sample, comprising: a source of abeam of radiation; a detector for detecting a beam of radiationreflected by or transmitted through the sample; an optical subsystem formanipulating the beam between the source and detector; and means fortranslating the optical subsystem along a first translation axisrelative to the sample to scan the beam across the sample; wherein thebeam from the source enters the subsystem on one side of the subsystemin a direction parallel to the first translation axis, and the beamreflected or transmitted exits the subsystem on the opposite side of thesubsystem in a direction parallel to the first translation axis.
 2. Anapparatus according to claim 1, further comprising a frequencyconversion device between the source and the sample.
 3. An apparatusaccording to claim 1 or 2, wherein a reference beam is provided whichenters the subsystem parallel to the beam of radiation from the source.4. An apparatus according to claim 1 or 2, further comprising a secondoptical subsystem, for manipulating the source beam between the sourceand detector, and the first subsystem being contained within the secondsubsystem.
 5. An apparatus according to claim 4, further comprisingmeans for translating the second optical subsystem relative to the fixedreference point to scan the source beam across the sample along a secondtranslation axis, wherein the source beam enters the second subsystem ina direction parallel to the direction of translation of the secondsubsystem.
 6. An apparatus according to claim 3, further comprising asecond optical subsystem for manipulating the source and reference beamsbetween the source and the detector, the first optical subsystem beingcontained within the second optical subsystem.
 7. An apparatus accordingto claim 6, further comprising means for translating the secondsubsystem along a second translation axis relative to the fixedreference point, to scan the source beam across the sample along thesecond translation axis, wherein source and reference beams each enterthe second subsystem in a direction parallel to the second translationaxis.
 8. An apparatus according to claim 4 to 7, wherein the detector iswithin the second subsystem.
 9. An apparatus according to claim 4 to 7,wherein the detector is outside the second subsystem, and the referencebeam and reflected or transmitted beam exit the second subsystemparallel to the direction of translation of the second subsystem.
 10. Anapparatus according to any of claims 4 to 9, wherein the first andsecond directions of translation are orthogonal.
 11. An apparatus forinvestigating a sample, comprising: a source of a beam of radiation; asource of a reference beam; an optical subsystem for manipulating thesource beam; means for translating the optical subsystem along a firsttranslation axis relative to the sample to scan the beam across thesample; and a detector for detecting the reflected or transmitted beam;wherein source and reference beams each enter the subsystem in adirection parallel to the first translation axis.
 12. An apparatusaccording to claim 11, wherein the detector is within the subsystem. 13.An apparatus according to claim 11, wherein the detector is outside thesubsystem, and the reference beam and reflected or transmitted beam exitthe subsystem parallel to the translation axis of the subsystem.
 14. Anapparatus for investigating a sample, comprising a source of a beam ofradiation, a source of a reference beam; an optical subsystem formanipulating the beam between the source and a detector; and means fortranslating the subsystem relative to the sample; wherein reference beamenters the subsystem through an electromagnetic radiation guide, one endbeing fixed with respect the source of the reference beam, the other theguide being fixed relative to the subsystem.
 15. An apparatus accordingto claim 14, wherein the source is also outside the subsystem and thesource beam enters the subsystem through an electromagnetic radiationguide.
 16. An apparatus according to claim 14 or 15, wherein eachelectromagnetic radiation guides is an optical fibre.
 17. An apparatusas claimed in any of the previous claims, wherein the detector comprisesa non-linear optical crystal.
 18. An apparatus according to any ofclaims 1 to 16, wherein the detector comprises a photoconductor.
 19. Anapparatus according to any of the previous claims, wherein the sample isirradiated with terahertz frequency radiation.
 20. An apparatusaccording to any preceding claim, wherein the source radiation has afrequency of between 0.1×10² Hz and 5×10⁴ Hz.
 21. An apparatus accordingto claim 3 or any claim dependent upon it, wherein the referenceradiation has a frequency of between 0.1×10² Hz and 5×10⁴ Hz.
 22. Anapparatus according to any of claims 3 or any claim dependent upon it,including a means for implanting a predetermined delay into one of saidsource and reference beams.
 23. An apparatus according to any of theprevious claims, wherein the incident beam is pulsed.
 24. A method ofinvestigating a sample, the method comprising: (a) irradiating a samplewith a beam of radiation, through a member which is substantiallytransparent to the beam of radiation; (b) detecting the radiationreflected from the sample; (c) irradiating the said member in theabsence of the sample; and (d) detecting radiation reflected from themember during step (c); (e) subtracting the signal measured by thedetector during step (d) from the signal measured by the detector duringstep (b).
 25. A method according to claim 24, wherein steps (c) and (d)are performed before steps (a) and (b).
 26. A method according to eitherof claims 24 or 25, wherein the sample and/or beam of radiation aremoved relative to one another and steps (a) to (b) are repeated in orderto scan a part of the sample, wherein the signal measured in step (d) issubtracted from each of the signals measured in step (b).
 27. A methodaccording to any of claims 24 to 26, wherein the irradiating beam ofradiation comprises a plurality of frequencies.
 28. A method accordingto claim 27, wherein step (e) is performed in the frequency domain. 29.A method according to any of claims 24 to 28, further comprising thesteps of: (f) irradiating a reference sample which has known reflectioncharacteristics with a beam of radiation, through the member; (g)detecting the radiation reflected from the reference sample; and (h)subtracting the signal measured by the detector during step (d) from thesignal measured by the detector during step (g); and (i) dividing theresult obtained in step (e) by the result derived in step (h).
 30. Amethod according to any of claims 24 to 38, wherein a reference sampleis placed on the member, such that the signal from the reference sampleand the member is measured in step (b).
 31. A method according to claim30, further comprising the steps of: (f) irradiating the member with abeam of radiation in the absence of both the reference sample andsample; (g) detecting the radiation reflected from the member; and (h)subtracting the signal measured by the detector during step (d) from thesignal measured by the detector during step (g); and (i) dividing theresult obtained in step (e) by the results derived in step (h).
 32. Anapparatus for investigating a sample, the apparatus comprising: anemitter for emitting a beam of radiation to irradiate the sample; adetector for detecting radiation reflected from the sample; a member,which is substantially transparent to radiation, located between thesample and the emitter and detector; means for subtracting apredetermined signal detected by the detector from a further signaldetected by the detector.
 33. A method of investigating a sample, themethod comprising the steps of: (a) irradiating a sample with a beam ofpulsed radiation comprising a plurality of frequencies; (b) measuringthe beam reflected by or transmitted through the sample and obtaining asignal representative of the measured beam; (c) multiplying the signalof step (b) in the frequency domain by the complex Fourier transform ofF(t), where F(t) is a non-zero function whose integral between limits t₁and t₂ is zero, where t₁ and t₂ are chosen on the basis of the timedelay introduced by the sample.
 34. A method according to claim 33,where${F(t)} = {{\frac{2}{\pi}\left\{ {\frac{^{{- 2}{(\frac{t}{\alpha})}^{2}}}{\alpha} - \frac{^{{- 2}{(\frac{t}{\beta})}^{2}}}{\beta}} \right\}}}$

and α and β are constants.
 35. A method according to clam 34, wherein ais substantially equal to the shortest pulse length of the beam ofpulsed radiation and β is set to be longer than the pulse length.
 36. Amethod according to claim 33, where:${F(t)} = {{\frac{2}{\pi}\left\{ {\frac{^{{- 2}{(\frac{t}{\alpha})}^{2}}}{\alpha} - \frac{1}{T}} \right\}}}$

and wherein α is a constant which is substantially equal to the shortestpulse length of the beam and T is substantially equal to the time whichit takes the beam of radiation to penetrate to the deepest point ofinterest in the sample.
 37. A method of investigating a sample,according to any of claims 33 to 36, wherein step (b) comprises: (b(i))irradiating a sample with a beam of radiation, through a member which issubstantially transparent to the beam of radiation; (b(ii)) detectingthe radiation reflected from the sample; (b(iii)) irradiating the saidmember in the absence of the sample; and (b(iv)) detecting radiationreflected from the member during step (b(iii)); (b(v)) subtracting thesignal measured by the detector during step (b(iv)) from the signalmeasured by the detector during step (b(ii)).
 38. A method ofinvestigating a sample, according to any of claims 33 to 36, whereinstep (b) comprises: (b(i)) irradiating the sample with a beam ofradiation, through a member which is substantially transparent to thebeam of radiation; (b(ii)) detecting radiation which is transmittedthrough or reflected from the sample; (b(iii)) irradiating the saidmember in the presence of a reference sample having known reflectionand/or transmission characteristics; and (b(iv)) detecting radiationwhich is reflected from or transmitted through the reference sampleduring step (b(iii)); (b(v)) dividing the signal measured by thedetector during step (b(ii)) with the signal measured by the detectorduring step (b(iv)).
 39. A method of investigating a sample, accordingto any of claims 33 to 36, wherein step (b) comprises: (b(i))irradiating the sample with a beam of radiation, through a member whichis substantially transparent to the beam of radiation; (b(ii)) detectingradiation which is transmitted through or reflected from the sample;(b(iii)) irradiating the said member in the presence of a referencesample having known reflection and/or transmission characteristics; and(b(iv)) detecting radiation which is reflected from or transmittedthrough the reference sample during step (b(iii)); (b(v)) irradiatingthe said member in the absence of the sample; (b(vi)) detectingradiation reflected from the member during step (b(v)); (b(vii))subtracting the signal measured by the detector during step (b(vi)) fromthe signal measured by the detector during step (b(ii)); (b(viii))subtracting the signal measured by the detector during step (b(vi)) fromthe signal measured by the detector during step (b(iv)); (b(ix))dividing the signal obtained in step(b(vii) with the signal obtained instep (b(viii)).
 40. A method of investigating a sample, according to anyof claims 33 to 36, wherein step (by comprises: (b(i)) irradiating thesample with a beam of radiation, through a member which is substantiallytransparent to the beam of radiation; (b(ii)) detecting radiation whichis transmitted through or reflected from the sample; (b(iii))irradiating the said member in the presence of a reference sample havingknown reflection and/or transmission characteristics; and (b(iv))detecting radiation which is reflected from or transmitted through thereference sample during step (b(iii)); (b(v)) irradiating the saidmember in the absence of the sample; (b(vi)) detecting radiationreflected from the member during step (b(v)); (b(vii)) subtracting thesignal measured by the detector during step (b(iv)) from the signalmeasured by the detector during step (b(ii)); (b(viii)) subtracting thesignal measured by the detector during step (b(iv)) from the signalmeasured by the detector during step (b(vi)); (b(ix)) dividing thesignal obtained in step(b(vii) with the signal obtained in step(b(viii)).
 41. An apparatus for investigating a sample with a beam ofradiation, the apparatus comprising an optically active element forirradiating the sample and/or detecting radiation reflected from thesample, the optically active element having at least two interfaces, thedistance between the interfaces being great enough such that the beam ofradiation takes longer to travel between the interfaces than it does totravel from the surface of the sample to the deepest point of interestwithin the sample.
 42. An apparatus according to claim 41, wherein theoptically active element is an emitter.
 43. An apparatus according toclaim 41, wherein the optically active element is a detector.
 44. Anapparatus according to claim 41, wherein the optically active element isa transceiver which is configured to act as both an emitter anddetector.
 45. An apparatus according to claim 42, further comprising asecond optically active element which is configured as a detector,wherein the second optically active element comprises at least twointerfaces, the distance between the interfaces being great enough suchthat the beam of radiation takes longer to travel between the interfacesthan it does to travel from the surface of the sample to the deepestpoint of interest within the sample.
 46. An apparatus according to anyof claims 41 to 45, wherein the optically active element is configuredas an emitter and emits the beam of radiation having the desiredfrequencies in response to irradiation by at least one input beam havinga different frequency.
 47. An apparatus according to claim 46, whereinthe emitter comprises a photo-conductive material.
 48. An apparatusaccording to claim 46, wherein the emitter comprises an opticallynon-linear material.
 49. An apparatus according to any preceding claim,wherein the optically active element is configured as a detector anddetects the beam of radiation having the desired frequencies using atleast one probe beam having a different frequency to that of the beam ofradiation.
 50. An apparatus according to claim 49, wherein the detectorcomprises an optically non-linear material which is configured to rotatethe polarisation of the probe beam in response to irradiation with thebeam of radiation.
 51. An apparatus according to claim 49, wherein thedetector comprises a photoconductive material and is configured togenerate a current within the photoconductive material in response toirradiation with both the beam of radiation and the probe beam.
 52. Anapparatus for investigating a sample, substantially as described withreference to the accompanying drawings.
 53. A method for investigating asample substantially as hereinbefore described with reference to theaccompanying drawings.